typedef tags<tag_file_name,tag_open_mode,tag_errno> file_err; throw my_error() << file_err(name,mode,errno);
Looks good.
I wonder why my_error() is required. I wonder if a globaly static object, such as "boost::exception()" might be even cleaner and more lightweight.
my_error is required so that you can catch my_error or other_error or something else. This is why I overreacted perhaps when Tobias said that boost::exception wants to be THE exception class. I feel very strongly that different failures should be reported by throwing exceptions of different types. Moreover, boost::exception makes it especially easy to classify your exception types because all of them can be empty structs; the only design the user has to come up with is a hierarchy. Emil Dotchevski